CDMA modulation techniques have been employed in communication systems to permit a large number of system users to communicate with one another. In a CDMA communication system, all communication channels are broadcast at a common broadband frequency. Each channel is differentiated by a unique spreading code. Prior to transmission, each information signal is modulated with a spreading code to convert the information signal into a broadband signal. A receiver demodulates the received broadband signal by combining the broadband signal with the corresponding spreading code to recover the information signal. The spreading code is typically a binary code. Since the same wide bandwidth is available to all users, information signals in other channels may appear as cochannel interference or noise when the received signal is demodulated by the spreading code.
A widely used CDMA communication system is disclosed in U.S. Pat. No. 4,901,307 ('307 patent), which is incorporated by reference herein. In this system, each information signal is modulated by a pseudo-random noise (PN) sequence before it is modulated for radio transmission. The modulation with the PN sequence can be regarded as a spreading process because it produces a very wide bandwidth compared with the original information signal. A communication system specified in standard IS-95 adopted by the Telecommunication Industry Association (TIA) is substantially similar to the communications system of the '307 patent. A more detailed description of the IS-95 communication system standard is provided in "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", Telecommunication Industry Association, Doc. No. TIA/EIA/IS-95 (1993), which is incorporated by reference herein.
In a forward communications link, from a base station or cell site to a mobile receiver unit the communication system of the '307 patent and the IS-95 standard substantially eliminate cochannel interference and improve bit energy to the noise density ratio E.sub.b /N.sub.O by modulating the information signals with Walsh orthogonal function sequences. To produce corresponding orthogonal information signals, these CDMA systems require that the forward link information signals be transmitted in a synchronized manner. A further improvement in the bit energy to noise density ratio E.sub.b /N.sub.O is achieved in the forward links of these systems by the use of coherent signal detection in the mobile unit receivers that is based on a pilot tracking signal transmitted by the cell-site.
However, in a reverse communications link of these systems, mobile units transmit data to a cell site using a different communication technique. The mobile units employ Walsh sequences in an M-ary signaling scheme with no synchronization when transmitting data signals. As a consequence, broadband signals transmitted in the reverse link are not orthogonal to one another and experience greater cochannel interference which results in a significant loss in the bit energy to noise density ratio E.sub.b /N.sub.O of the signal received by the cell-site. Further, since the mobile units do not transmit a pilot tracking signal the corresponding cell-site does not employ coherent detection resulting in a further loss in the bit energy to noise density ratio E.sub.b /N.sub.O. As a consequence, the maximum data transmission rate and corresponding bit error rate of the communication systems are limited by this reverse link communication technique.
A recognized need exists for a CDMA system that can achieve greater data transmission rates with the capability of reduced bit error rates. Such a system should employ coherent signal detection in the forward and reverse communications links and also enable information signals in the reverse link to be transmitted in a synchronous or nonsynchronous manner depending upon the mobile unit environment.